In a large-scale computer system, a logical volume which combines a plurality of disk devices (called a virtual volume as well) may be defined. In a logical volume, correspondence relationships between a logical block number used by an application for accessing (logical block number), and one disk device among a plurality of disk devices and a block number of the disk device (physical block number) are defined. Thus, the application can access a block by designating the logical block number and uniquely identifying the corresponding disk device and physical block number. A Redundant Array of Independent Disks (RAID) system having a plurality of disk devices may be used as one disk device to allocate to a logical volume.
In a system using such logical volume, the amount of data stored into a logical volume is gradually increased after long operation. Then, there arises a need to extend storage capacity. When storage capacity of a logical volume is extended, a disk device is additionally allocated to the logical volume. By increasing disk devices to be allocated to the logical volume, the storage capacity provided to an application by the logical volume increases as well.
However, in order to extend a storage area of the logical volume, the use of the logical volume needs to be temporarily discontinued and a new storage area needs to be defined. In this case, the operation of logical volume is temporarily discontinued. Thus, a technique is considered that allows extending a storage area without discontinuing use of a logical volume. For example, a plurality of volumes (internal logical volume) and logical volume recognized externally (external logical volume) can be provided. Then the correspondence relationship between the internal logical volume and the external logical volume is redefined during operation. This enables extension of the logical volume from the perspective of a computing machine.
Data volume in a management table increases as storage capacity of the logical volume increases, when allocation of disk devices to a logical volume is managed by the table. For example, when disk devices allocated to the logical volume are increased in order to increase storage capacity of the logical volume, information to manage the correspondence relationship increases as well. The management table needs to be stored in a memory during operation. Therefore, the increase in capacity of the management table results in increase in usage of the memory resource.
A technique to reduce the volume of the memory resource required for storing a management table has been considered. For instance, all management data in the management table can be allocated to a disk drive, only storing the required part in the memory each time, so the usage of memory is reduced.
By applying the above technique, disk devices allocated to a logical volume can be reconfigured. This reconfiguration function allows replacing a disk device allocated to the logical volume with another disk device. For example, a plurality of disk devices allocated to a logical volume can be replaced with a single disk device with larger storage capacity. This replacement reduces the number of disk devices allocated to the logical volume, thereby reducing data volume of the management table.
There may be a case where only a limited number of disk devices can be allocated to a logical volume even if reducing usage of memory for storing the management table by applying the above technique is possible. In this case, as a system bloats, disk devices allocated to a logical volume need to be replaced with disk devices with larger storage capacity to reduce the number of disk devices.
However, a large amount of stored data needs to be copied when disk devices allocated to a logical volume are replaced. Copying all data in the disk devices may increase the amount of data on communication paths among disk devices. This lowers the efficiency of other data communication in the communication path. Frequent input and output to the disk device that is being copied deteriorates access efficiency to relevant disk devices for providing services under operation.